Biofunctionalized silk nanofibers to repair the optic nerve
(Nanowerk Spotlight) A main difference between central and peripheral nervous system is the lack of regeneration after a neurotrauma, leading to severe and irreversible handicaps. While biomaterials have been developed to aid the regeneration of peripheral nerves, the repair of central nerves such as the optic nerve or nerve cells in the spinal cord remain a major challenge for scientists.
"Since the first regenerative trial, achieved by J.F. Tello a hundred years ago (Trab. Lab. Invest. Biol. Univ. Madrid 1911, 9, 123-159), our knowledge in the discrepancies between the two system have dramatically increased, without leading however to realistic therapeutic strategies," Thomas Claudepierre, a professor in the Faculty of Medicine at the University of Leipzig, tells Nanowerk. "This is mainly due to the multifactorial cascade of events during the neurodegenerative process following a trauma in the central nervous system."
As Claudepierre explains, in the case of damage to the optic nerve, axons of neurons (namely the retinal ganglion cells or RGC) that form the optic nerve first degenerate following a nerve crush or cut. Surrounding glia cells will lose their orientation and form a glia scar at the site of lesion, modifying profoundly the matrix deposition and the global structure orientation. Glia cell undergo gliosis a proliferative state where they secrete survival but also noxious factors that will affect the neuroretina. The RGC fail to re-grow their axon due to lack of orientation clues for growth cone and massively die.
Retinal ganglion cells (labelled in red with beta III tubulin antibody) growing on electrospun silk fibers, neurites are extending mainly in close contact with silk that are seen in contrast phase. (Image: Dr. Thomas Claudepierre, University of Leipzig)
"The goal of our study is to test new biomaterials that will provide a permissive growth support that is orientated and can be functionalize with survival factors for RGC (growth factors matrix molecules etc), and can ultimately develop as a 3D nerve guide that can be implanted at a site of a nerve lesion to regenerate the traumatized axons," says Claudepierre.
He points out that, while progress has been made in generating artificial nerve-regrowth systems using different approaches, no nerve guide has yet emerged that can: 1) physically align the growth of regenerating nerves; 2) switch neurons to a regenerative state and promote nerve growth and axonal adhesion; 3) eliminate scar-tissue formation; 4) avoid rejection by the body; 5) not swell to impinge on the nerve; 6) degrade to form by-products that are nontoxic or non-inflammatory; and 7) be easily handled by surgeons.
The researchers selected silk as it is a highly biocompatible material used for years in surgery. It can be prepared in purified orientated fibers using electrospinning methods. Last but not least it can be biofunctionalized during the spinning process.
"We selected growth factors known to act on RGC survival and neurite elongation" says Claudepierre. "We use purified primary RGC isolated using the immunopanning method from newborn rodents. Our in vitro model focus therefore on the first step of the trauma: can axotomized RGC axon regenerate along a silk guide containing survival molecules?"
In their recent work, the research team demonstrates that electrospun silk fiber is fully biocompatible and allow RGC survival at their contact; that RGC growth cones preferentially follow the silk fibers and that RGCs extend robust neurites expressing the axonal markers GAP43.
"Moreover" as Claudepierre notes, "we found the same rate of survival, regeneration and axon differentiation when growth factors were embedded in the silk fibers and not added to the medium. The embedded process also conferred stability to growth factors that are otherwise easily degraded in the medium."
"In our study, we provide evidence that silk is a highly biocompatible material in regenerative neuroscience, it guides the growth cone and can topically provide surviving molecules that are protected from degradation and can act therefore over a long period of time, compatible with the duration of a regenerative process in the central nervous system," he sums up the conclusion of the team's work.
The researchers' goal is now to demonstrate that silk guide can also provide orientation clues for glia cells and can therefore reduce the amount of glia scar, maintaining gliosis under control.
"We are actually testing the behavior of primary glia cells at silk contact and perform co-culture of glia and RGC on this biomaterial," Claudepierre describes the current thrust of their work. "We will test the effect of various matrix molecules on the guidance of the growth cone and orientation of glia cells along the silk guide. Once we obtained the best combination of surviving factors and guidance signals we will develop a 3D guide that we will implant on rodent at the site of an optic nerve lesion to promote a massive axon grow that will be able to cross the lesion site. The final challenge would be to reconnect the axon to their physiological target in the brain, restoring the normal cartography of retinal projections."
Such strategies, if successful, could lead to medical applications in the treatment of traumatic optic neuropathies and would be also useful to handle other central nervous system lesions.